The mechanism of ethylene biosynthesis and its role in horticultural crops and regulation and function of ethylene synthesis during ripening and senescence

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Ethylene plays an essential role in growth, development ; differentiation, and senescence of plants. In horticultural crops ethylene is closely involved in the mechanism of ripening and senescence. On the other hand, ethylene is rapidly produced in response to various kinds of stresses such as wounding and disease caused by infection with microorganisms. (1)Ethylene biosynthesis and its regulation during senescence of broccoli florets (flower buds): Broccoli is harvested while florets are still green and immature which are covered with green sepals. After harvest senescence proceeds rapidly in florets at ambient temperatures in which yellowing progresses due to chlorophyll degradation. The high levels of ascorbic acid (vitamin C) rapidly decline after harvest during senescence. The rate of ethylene production increased during senescence which was paralleled by the increase in ACC oxidase activity and its gene expression. The rate of ethylene biosynthesis in broccoli florets was regulat … More ed by the gene expression of ACC synthase and ACC oxidase after harvest. Harvesting broccoli causes the cessation of the transport of water, hormones, sugars, amino acids, and minerals from the mother plant and together with wounding by cutting the stem may result in triggering or accelerating senescence of the young flower organ of florets. (2)Ethylene biosynthesis and its mechanism in sweet potato root tissue infected by black rot fungus (Ceratocystis fimbriata): A large amount of ethylene is produced in sweet potato roots upon infection by black rot fungus. The pathway of ethylene biosynthesis was independent on methionine-ACC pathway that is widely operating in higher plants. We have found the new pathway for the synthesis of ethylene in sweet potato root tissue infected by the pathogen. The mechanism of the novel pathway is summarized as follows. In response to infection with the fungus, phospholipase A_2 is activated, liberating linolenic acid, which is hydroperoxidized by lipoxygeriase in the host tissue. Then, ethylradical is cleaved from the peroxidized linolenic acid, which is oxidized by cupric ion to yield ethylene. Ethylene produced may be involved in the activation of metabolism in sweet potato root tissue infected by the pathogen. Less